Plant growth modifiers include chemicals designed to alter and regulate the growth habit of plants (growth regulators) and those designed to so disturb their normal development that the plants are destroyed (herbicides). To be useful, both kinds of substances, must be effective when applied in extremely low concentration.
Growth Regulators
Plant growth regulators are used to improve yields, increase or decrease seed germination, cause early or late maturation of plants, improve quality, accelerate the ripening of fruits and grains, increase yields, halt undesirable flowering, produce fewer but larger fruits by thinning out blossoms, prevent preharvest drop, cause fruit drop when desired, produce seedless fruit, and control sprouting of stored vegetables. It is expected that the market for these substances (now 5 to 10 percent of the agrichemical market) will eventually exceed by present herbicide market. The chemicals used are very diverse, which is not surprising since the results are also diverse. A list of the chemicals in large scale use is available, and an extensive compilation of chemicals used earlier is given by Leopold.
The first substances to become commercially important were 3-indoleacetic acid, naphthalene acetic acid, and indolebutyric acid. These substance stimulate root growth, inhibit preharvest fruit drop of apples, and can cause fruit to set in the absence of pollinator insects. Naphtalene acetic acid also can be used at the proper time and concentration to thin fruit, allowing to the remainder to develop to superior size. Flowering in pineapples can be induced by using as little as 0.12 g/m2 of 2-chloroethyl phosphonic acid (Ethepon). Floral production in fruit trees is controlled by spraying with succinic trade names of a broad variety of plant growth regulators.